TECHNICAL FIELD
[0001] The present invention relates to a copper electrolytic solution used in manufacturing
electrolytic copper foils and 2-layer flexible substrates and other printed wiring
boards, and relates particularly to a copper electrolytic solution used in manufacturing
electrolytic copper foils with excellent elongation and tensile strength that allow
fine patterning and 2-layer flexible substrates.
BACKGROUND ART
[0002] An electrolytic copper foil is generally produced as follows. A rotating metal cathode
drum with a polished surface is used along with an insoluble metal anode that surrounds
said cathode drum and is disposed at a position substantially corresponding to the
lower half of said cathode drum, a copper electrolytic solution is allowed to flow
between the cathode drum and the anode, a potential differential is provided between
these to electrodeposit copper to the cathode drum, and the electrodeposited copper
is peeled away from the cathode drum at the point of reaching a specific thickness,
so that a copper foil is produced continuously.
[0003] A copper foil obtained in this way is generally called a raw foil, and after this
it is subjected to a number of surface treatments and used for printed wiring boards
and so forth.
[0004] Fig. 1 is a simplified diagram of a conventional apparatus for producing a copper
foil. This electrolytic copper foil production apparatus has a cathode drum 1 installed
in an electrolysis bath containing electrolytic solution. This cathode drum 1 is designed
to rotate while being partially submerged (substantially the lower half) in the electrolytic
solution.
[0005] An insoluble anode 2 is provided so as to surround the outer peripheral lower half
of this cathode drum 1. A specific gap 3 is maintained between the cathode drum 1
and the anode 2, and an electrolytic solution is allowed to flow through this gap.
Two anode plates are disposed in the apparatus shown in Fig. 1.
[0006] With the apparatus in Fig. 1, the electrolytic solution is supplied from below, and
this electrolytic solution goes through the gap 3 between the cathode drum 1 and the
anode 2, overflows from the top edge of the anode 2, and is then recirculated. A rectifier
is interposed between the cathode drum 1 and the anode 2 so that a specific voltage
can be maintained between the two components.
[0007] As the cathode drum 1 rotates, the thickness of the copper electrodeposited from
the electrolytic solution increases. When at least a certain thickness is reached,
this raw foil 4 is peeled away and continuously taken up. A raw foil produced in this
manner is adjusted for thickness by varying the distance between the cathode drum
1 and the anode 2, the flow rate of the supplied electrolytic solution, or the amount
of electricity supplied.
[0008] A copper foil produced with an electrolytic copper foil producing apparatus such
as this has a mirror surface on the side touching the cathode drum, but the opposite
side is a rough surface with bumps and pits. Problems encountered with ordinary electrolysis
are that the bumps and pits on the rough side are severe, undercutting tends to occur
during etching, and fine patterning is difficult.
[0009] On the one hand, as the density on printed wiring boards has steadily risen, there
has more recently been a need for a copper foil that can be more finely patterned
as circuit width decreases and multilayer circuits are produced. This fine patterning
requires a copper foil that has a good etching rate and uniform solubility, that is,
a copper foil with excellent etching characteristics.
[0010] Meanwhile, the properties required of copper foils for printed wiring boards include
not only elongation at room temperature but also elongation properties to prevent
cracking due to temperature stress, as well as high tensile stress to maintain the
dimensional stability of the printed wiring board.
[0011] = However, a copper foil with a highly irregular rough surface is wholly unsuited
to fine patterning as described above. Ways are therefore being studied on lowering
the profile of the rough surface. It is known that the profile can be lowered by adding
large quantities of glue or thiourea to the electrolytic solution.
However, the problem with such additives is that they dramatically lower the elongation
percentage, greatly detracting from the foil's properties as a copper foil for printed
wiring boards.
[0012] 2-layer flexible substrates have gained attention as substrates for preparing flexible
wiring boards. Because in a 2-layer flexible substrate a copper conductor layer is
provided directly on an insulating film without an adhesive, the substrate itself
can advantageously be kept thin and the thickness of the copper conductor layer can
be adjusted at will before adhesion. The normal method of manufacturing such a 2-layer
flexible substrate is to form an underlying metallic layer by dry plating on the insulating
film, and then electroplating copper on top. However, the underlying metallic layer
obtained in this way contains numerous pinholes, resulting in exposure of the insulating
film, and in the case of a thin copper conductor layer the areas exposed by the pinholes
are not filled in and pinholes occur on the surface of the copper conductor layer,
leading to wiring defects. As a means of solving this problem, Patent Document 1 for
example describes a 2-layer flexible substrate manufacturing method in which an underlying
metallic layer is formed on an insulating film by a dry plating process, a primary
electrolytic copper plating coating layer is formed on the underlying metallic layer
and treated with an alkali solution, after which an electroless copper plating coating
is adhered, and finally a secondary electrolytic copper plating coating layer is formed.
However, this method involves complex steps.
Patent Document 1: Japanese Patent Publication No.
H10-193505
DISCLOSURE OF THE INVENTION
PROBLEMS THAT THE INVENTION IS TO SOLVE
[0013] It is an object of the present invention to provide a low profile electrolytic copper
foil with low surface roughness at the rough surface side (opposite side from the
glossy side) in the electrolytic copper foil manufacture using a cathode drums, and
in particular to provide an electrolytic copper foil with excellent elongation and
tensile strength that allows fine patterning.
Another object is to provide a copper electrolytic solution capable of uniform copper
plating without pinholes on a 2-layer flexible substrate.
MEANS FOR SOLVING THE PROBLEMS
[0014] The inventors discovered that an electrolytic copper foil with excellent elongation
and tensile strength that allows fine patterning and a 2-layer flexible substrate
having a uniform copper plating without pinholes could be obtained by adding to the
electrolytic solution an additive optimal for obtaining a low profile.
[0015] Based on this finding, the inventors perfected the present invention upon discovering
that an electrolytic copper foil with excellent elongation and tensile strength that
allows fine patterning can be obtained by electrolysis using a copper electrolytic
solution containing a compound with a specific skeleton in an electrolytic copper
foil manufacturing method in which a copper electrolytic solution is made to flow
between a cathode drum and an anode to electrodeposit copper on the cathode drum,
after which the electrodeposited copper foil is peeled from the cathode drum to manufacture
a continuous copper foil. The inventors also discovered that in a method for manufacturing
a 2-layer flexible substrate, a 2-layer flexible substrate having a uniform copper
plating layer without pinholes could be obtained by first forming an underlying metal
layer on an insulating film by dry plating using at least one selected from the group
consisting of nickel, nickel alloy, chrome, cobalt, cobalt alloy, copper and copper
alloy, and then plating using a copper electrolytic solution containing a compound
having a specific skeleton.
[0016] That is, the present invention consists of the following.
- (1) A copper electrolytic solution containing as an additive a compound having a specific
skeleton represented by General Formula (1) below, which is obtained by an addition
reaction in which water is added to a compound having in a molecule at least one epoxy
group:
wherein A is an epoxy compound residue and n is an integer of 1 or more.
[0017] (2) The copper electrolytic solution according to (1) above, wherein the epoxy compound
residue A of the aforementioned compound having a specific skeleton has a linear ether
bond.
[0018] (3) A copper electrolytic solution according to (1) or (2) above, wherein the aforementioned
compound having a specific skeleton includes any of compounds represented by chemical
formulae (2) through (9) below:
[0019]
[0020]
[0021]
wherein n is an integer of 1 to 5.
[0022]
[0023]
[0024]
wherein n is an integer of 1 to 22.
[0025]
wherein n is an integer of 1 to 3.
[0026] (4) The copper electrolytic solution according to any one of (1) through (3) above,
wherein the aforementioned copper electrolytic solution contains an organic sulfur
compound.
[0027] (5) The copper electrolytic solution according to (4) above, wherein the aforementioned
organic sulfur compound is a compound represented by General Formula (10) or (11)
below:
X-R
1-(s)
n-R
2-Y (10)
R
4-S-R
3-SO
3Z (11)
wherein, in general formulae (10) and (11), R
1, R
2 and R
3 are alkylene groups with 1 through 8 carbon atoms, R
4 is selected from the group consisting of hydrogen and
X is selected from the group consisting of hydrogen, a sulfonic acid group, a phosphonic
acid group, and an alkali metal salt group or ammonium salt group of sulfonic acid
or phosphonic acid, Y is selected from the group consisting of a sulfonic acid group,
a phosphonic acid group, and an alkali metal salt group of sulfonic acid or phosphonic
acid, Z indicates hydrogen or an alkali metal, and n is 2 or 3.
[0028] (6) An electrolytic copper foil manufactured using the copper electrolytic solution
according to any one of (1) through (5) above.
(7) A copper clad laminate formed using the electrolytic copper foil according to
(6) above.
(8) A printed wiring board manufactured using the copper electrolytic solution according
to any one of (1) through (5) above.
(9) A printed wiring board wherein the printed wiring board according to (8) above
is a 2-layer flexible substrate.
EFFECTS OF THE INVENTION
[0029] The copper electrolytic solution of the present invention having a compound with
a specific skeleton and also an organic sulfur compound added thereto is extremely
effective for lowering the profile of the resulting electrolytic copper foil and 2-layer
flexible substrate, effectively maintains elongation properties in the copper foil,
and also provides high tensile strength.
BEST MODE FOR CARRYING OUT THE INVENTION
[0030] In the present invention, it is vital that the compound with the specific skeleton
represented by General Formula (1) above, which is obtained by an addition reaction
in which water is added to a compound having in the molecule one or more epoxy groups,
be present in the electrolytic solution.
The compound with the specific skeleton represented by General Formula (1) above is
synthesized by the addition reaction represented by the following reaction formula.
That is, it can be manufactured by mixing a compound having one or more epoxy groups
in the molecule with water and reacting them for about 10 minutes through 48 hours
at 50 through 100°C:
[0031]
wherein A is an epoxy residue and n is an integer of 1 or more.
[0032] The compound having a specific skeleton is preferably a compound having a linear
ether bond in epoxy compound residue A. A compound having one of the structural formulae
(2) through (9) below is preferred as the compound having a linear ether bond in epoxy
compound residue A, and in formulae (2) through (9) epoxy compound residue A is as
follows:
[0033]
[0034]
[0035]
[0036]
wherein n is an integer of 1 to 5.)
[0037]
[0038]
[0039]
wherein n is an integer of 1 to 22.
[0040]
wherein n is an integer of 1 to 3.
[0041] An organic sulfur compound is preferably added to the aforementioned copper electrolytic
solution. The organic sulfur compound is preferably a compound having as its structural
formula General Formula (10) or (11) above.
The following are examples of the organic sulfur compound represented by General Formula
(10) above, and can be used by preference.
H
2O
3P-(CH
2)
3-S-S-(CH
2)
3-PO
3H
2
HO
3S-(CH
2)
4-S-S-(CH
2)
4-SO
3H
NaO
3S-(CH
2)
3-S-S-(CH
2)
3-SO
3Na
HO
3S-(CH
2)
2-S-S-(CH
2)
2-SO
3H
CH
3-S-S-CH
2-SO
3H
NaO
3S-(CH
2)
3-S-S-S-(CH
2)
3-SO
3Na
(CH
3)
2CH-S-S-(CH
2)
2-SO
3H
[0042] The following are examples of the organic sulfur compound represented by General
Formula (11) above, and can be used by preference.
[0043] The ratio of the aforementioned compound having a specific skeleton to the organic
sulfur compound in the copper electrolytic solution is preferably between 1:50 and
100:1 or more preferably between 1:10 and 50:1 by weight. The concentration of the
compound having a specific skeleton in the copper electrolytic solution is preferably
1 through 1000 ppm or more preferably 1 through 200 ppm.
[0044] The copper electrolytic solution of the present invention can contain as additives
those used in ordinary acidic copper electrolytic solutions in addition to the aforementioned
compound having a specific skeleton and organic sulfur compound, and known additives
such as polyethylene glycol, polypropylene glycol and other polyether compounds, polyethylenimine,
phenazine dye, glue, cellulose and the like can be added.
[0045] For the plating conditions, a plating temperature of 50 through o 65°C and a current
density of 40 through 150 A/dm
2 is preferred for copper foil manufacture, while in the case of a 2-layer flexible
substrate a plating temperature of 25 through 60°C and a current density of 1 through
50 A/cm
2 is preferred.
A copper clad laminate obtained by laminating the electrolytic copper foil of the
present invention is a copper clad laminate with excellent elongation and tensile
strength.
Examples
[0046] The present invention is explained in more detail below using examples.
Synthesis Example 1 of compound having specific skeleton
[0047] 10.0 g (epoxy groups 0.0544 mol) of the epoxy compound represented by the following
chemical formula (Denacol EX-521, manufactured by Nagase Chemitex Corp.) and 40.0
g of pure water were placed in a triangular flask and reacted for 24 hours at 85°C
using a cooling tube having dry ice-methanol as the cooling medium, to obtain the
following compound (compound of Formula (5) above, n = 3).
[0048]
[0049] The
13C-NMR spectrum of the resulting compound is shown in Figure 2. The
13C-NMR spectrum of the raw material epoxy resin (Denacol EX-521, manufactured by Nagase
Chemitex Corp.) is also shown for comparison in Figure 3. As clear from Figures 2
and 3, peaks at 52 ppm and 45 ppm attributed to the epoxy groups disappeared from
the resulting compound and this indicates the cleavage of the epoxy groups.
Synthesis Examples 2 through 6 of compounds having specific skeletons
[0050] The following compounds having specific skeletons were synthesized as in Synthesis
Example 1 except that the following compounds were used in place of the epoxy resin
(Denacol EX-521, manufactured by Nagase Chemitex Corp.) used in Synthesis Example
1 of a compound having a specific skeleton.
Synthesis Example 2: Compound of Formula (5) above (n = 1) (raw material epoxy resin:
Decanol EX-421, manufactured by Nagase Chemitex Corp.)
Synthesis Example 3: Compound of Formula (2) above (raw material epoxy resin: Decanol
EX-614B, manufactured by Nagase Chemitex Corp.)
Synthesis example 4: Compound of Formula (8) above (n ≅ 13) (raw material epoxy resin:
Decanol EX-841, manufactured by Nagase Chemitex Corp.)
Synthesis Example 5: Mixture of compounds of Formulae (3) and (4) above (raw material
epoxy resin: Decanol EX-313, manufactured by Nagase Chemitex Corp.)
Synthesis Example 6: Compound of Formula (9) above (n ≅ 3) (raw material epoxy resin:
Decanol EX-920, manufactured by Nagase Chemitex Corp.)
Examples 1 through 13 and Comparative Examples 1 and 2
[0051] 35 µm electrolytic copper foils were manufactured at a current density of 90 A/dm
2 using the electrolytic copper foil manufacturing device shown in Figure 1. The compositions
of the electrolytic solutions were as follows, with the additives added in the amounts
shown in Table 1.
Cu: |
90 g/L |
H2SO4: |
80 g/L |
Cl: |
60 ppm |
Liquid temperature: |
55 through 57°C |
Additive A: |
bis(3-sulphopxopyl)disulfide disodium salt (SPS, manufactured by Raschig) |
Additive B: |
3-mexcapto-1-propanesulfonate sodium salt (Raschig MPS) |
Additive C: |
compounds having specific skeletons obtained in aforementioned synthesis examples |
C1: |
Compound of Synthesis Example 1 |
C2: |
Compound of synthesis Example 2 |
C3: |
Compound of Synthesis Example 3 |
C4: |
Compound of Synthesis Example 4 |
C5: |
Compound of Synthesis Example 5 |
C6: |
Compound of Synthesis Example 6 |
[0052] The surface roughness Rz (µm) of the resulting electrolytic copper foils was measured
in accordance with JIS B 0601 and the elongation (%) at room temperature and the tensile
strength (kgf/mm
2) at room temperature in accordance with IPC-TM650. The results are shown in Table
1.
[Table 1]
|
Additives (ppm) |
Rz
(µm) |
Room temp.
elongation (%) |
Room temp.
tensile strength
(kgf/mm2) |
A |
B |
C |
C1 |
C2 |
c3 |
C4 |
C5 |
c6 |
Example 1 |
50 |
0 |
50 |
0 |
0 |
0 |
0 |
0 |
1.70 |
6.20 |
58.1 |
Example 2 |
50 |
0 |
0 |
50 |
0 |
0 |
0 |
0 |
1.68 |
5.40 |
55.5 |
Example 3 |
50 |
0 |
0 |
0 |
50 |
0 |
0 |
0 |
1.55 |
6.11 |
59.2 |
Example 4 |
50 |
0 |
0 |
0 |
0 |
50 |
0 |
0 |
1.72 |
5.50 |
62.0 |
Example 5 |
50 |
0 |
0 |
0 |
0 |
0 |
50 |
0 |
1.85 |
5.20 |
52.0 |
Example 6 |
50 |
0 |
0 |
0 |
0 |
0 |
0 |
50 |
1.95 |
6.03 |
58.6 |
Example 7 |
0 |
50 |
50 |
0 |
00 |
0 |
0 |
0 |
1.68 |
6.10 |
57.5 |
Example 8 |
0 |
50 |
0 |
50 |
0 |
0 |
0 |
0 |
1.65 |
5.52 |
55.5 |
Example 9 |
0 |
50 |
0 |
0 |
50 |
0 |
0 |
0 |
1.58 |
6.10 |
61.0 |
Example 10 |
0 |
50 |
0 |
0 |
0 |
50 |
0 |
0 |
1.90 |
5.35 |
62.5 |
Example 11 |
0 |
50 |
0 |
0 |
0 |
0 |
50 |
0 |
1.80 |
5.25 |
51.5 |
Example 12 |
0 |
50 |
0 |
0 |
0 |
0 |
0 |
50 |
1.92 |
6.13 |
59.2 |
Example 13 |
0 |
0 |
50 |
0 |
0 |
0 |
0 |
0 |
2.20 |
5.10 |
72.0 |
Comparative Example 1 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
5.80 |
8.90 |
37.9 |
Comparative Example 2 |
100 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
5.30 |
0.21 |
10.3 |
[0053] As shown in Table 1 above, in Examples 1 through 13 in which a compound having a
specific skeleton was added the surface roughness Rz was in the range of 1.55 through
2.20 µm while the elongation at room temperature was 5.10 through 6.20% and the tensile
strength at room temperature was 51.5 through 72.0 kgf/mm
2. Thus, despite the dramatic low profile achieved in these examples the elongation
at room temperature and tensile strength at room temperature were equal to or greater
than those achieved in Comparative Example 1, in which the compound having a specific
skeleton of the present invention was not added. By contrast, a low profile was not
achieved in Comparative Examples 1 and 2 in which the compound having a specific skeleton
of the present invention was not added.
Examples 14 through 19 and Comparative Examples 3 and 4
[0054] Polyimide films were electroplated under the following plating conditions to have
roughly a 9 µm thick copper coating. The additives were added in the amounts shown
in Table 2.
Liquid content:About 800 ml |
Anode: |
Lead electrode |
Cathode: |
Rotating electrode wrapped in polyimide film |
Polyimide film: |
37.5 µm thick Kapton E, manufactured |
by Dupont, coated with 10 nm NiCr + 2000 Å Cu by sputtering |
Plating temperature: |
50°C |
Current time: |
1220 As |
Current density: |
changing of 5 → 10 → 20 → 30 A/dm2 |
Flow velocity: |
190 r.p.m. |
Cu: |
70 g/L |
H2SO4: |
60 g/L |
C1: |
75 ppm |
Additive A: |
bis(3-sulphopropyl)disulfide disodium |
|
salt (Raschig SPS) |
Additive C: |
Compounds having specific skeletons obtained in aforementioned synthesis examples |
C1: |
Compound of Synthesis Example 1 |
C2: |
Compound of Synthesis Example 2 |
C3: |
Compound of Synthesis Example 3 |
C4: |
Compound of Synthesis Example 4 |
C5: |
Compound of Synthesis Example 5 |
C6: |
Compound of Synthesis Example 6 |
[0055] The surface roughness Rz (µm) (10-point average roughness) and surface roughness
Ra (µm) (arithmetic average roughness) of each of the obtained 2-layer flexible substrates
were measured in accordance with JIS B 0601. The planting surface was also observed
for plating defects by optical microscopy and SEM. The results are shown in Table
2.
[0056]
[Table 2]
|
Additive
(ppm) A |
Additive C (ppm) |
Rz
(µm) |
Defects |
Appearance |
Ra
(µm) |
|
C1 |
C2 |
C3 |
C4 |
C5 |
C6 |
Example 14 |
50 |
50 |
0 |
0 |
0 |
0 |
0 |
1.78 |
no |
semi-gloss |
0.19 |
Example 15 |
50 |
0 |
50 |
0 |
0 |
0 |
0 |
1.69 |
no |
semi-gloss |
0.17 |
Example 16 |
50 |
0 |
0 |
50 |
0 |
0 |
o |
2.18 |
no |
semi-gloss |
0.31 |
Example 17 |
50 |
0 |
0 |
0 |
50 |
0 |
0 |
1.73 |
no |
semi-gloss |
0.19 |
Example 18 |
50 |
0 |
0 |
0 |
0 |
50 |
0 |
1.80 |
no |
semi-gloss |
0.20 |
Example 19 |
50 |
0 |
0 |
0 |
0 |
0 |
50 |
1.63 |
no |
semi-gloss |
0.15 |
Comparative Example 3 |
50 |
0 |
0 |
0 |
0 |
0 |
0 |
6.63 |
yes |
no gloss |
1.02 |
Comparative Example 4 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
7.32 |
yes |
no gloss |
1.10 |
[0057] As shown in Table 2, Examples 14 through 19 in which the compound having a skeleton
structure of the present invention was added all exhibited semi-gloss, with surface
roughness Rz in the range of 1.63 through 2.18 µm and Ra in the range of 0.15 to 0.31
µm and no defects, and thus appeared suited to fine patterning.
BRIEF DESCRIPTION OF THE DRAWINGS
[0058]
Figure 1 shows one example of an electrolytic copper foil manufacturing device.
Figure 2 shows the 13C-NMR spectrum of a compound obtained in Synthesis Example 1 of a compound having
a specific skeleton.
Figure 3 shows the 13C-NMR spectrum of the epoxy compound (Decanol EX-521, manufactured by Nagase Chemitex
Corp..) used in Synthesis Example 1 of a compound having a specific skeleton.
Explanation of Reference Numerals
[0059]
1: cathode drum
2: anode
3: gap
4: raw foil
1. A method for manufacturing an electrolytic copper foil using a copper electrolytic
solution containing as an additive a compound having a specific skeleton represented
by General Formula (1) below, which is obtained by an addition reaction in which water
is added to a compound having in a molecule at least one epoxy group:
wherein A is an epoxy compound residue and n is an integer 1 or more.
2. The method according to Claim 1, wherein the epoxy compound residue A of said compound
having the specific skeleton has a linear ether bond.
4. The method according to any one of Claims 1 through 3, wherein said copper electrolytic
solution contains an organic sulfur compound.
5. The method according to Claim 4, wherein said organic sulfur compound is a compound
represented by General Formula (10) or (11) below:
X-R
1-(S)
n-R
2-Y (10)
R
4-S-R
3-SO
3Z (11)
wherein in formulae (10) and (11), R
1, R
2 and R
3 are alkylene groups with 1 through 8 carbon atoms, R
4 is selected from the group consisting of hydrogen and
X is selected from the group consisting of hydrogen, a sulfonic acid group, a phosphonic
acid group, and an alkali metal salt group or ammonium salt group of sulfonic acid
or phosphonic acid, Y is selected from the group consisting of a sulfonic acid group,
a phosphonic acid group, and an alkali metal salt group of sulfonic acid or phosphonic
acid, Z indicates hydrogen or an alkali metal, and n is 2 or 3.
6. An electrolytic copper foil manufactured by the method according to any one of claims
1 through 5.
7. The electrolytic copper foil according to claim 6 wherein the electrolytic copper
foil has a surface roughness Rz, measured in accordance with JIS B 0601, of 1.55 to
2.20 µm, an elongation, measured in accordance with IPC-TM650 at room temperature,
of 5.10 to 6.20% and a tensile strength, measured in accordance with IPC-TM650 at
room temperature, of 51.5 to 72.0 kgf/mm2.
8. A method for forming a copper clad laminate using the electrolytic copper foil according
to Claim 6 or 7.
9. A copper clad laminate formed by the method to Claim 8.
10. A method for manufacturing a printed wiring board using the copper electrolytic solution
described in any one of Claims 1 through 5.
11. The method according to Claim 10 wherein the printed wiring board is a 2-layer flexible
substrate.
12. A printed wiring board manufactured by the method according to claim 10 or 11.
13. The printed wiring board according to claim 12, wherein the printed wiring board is
a 2-layer flexible substrate which has a surface roughness Rz, measured in accordance
with JIS B 0601, of 1.63 to 2.18 µm and a surface roughness Ra, measured in accordance
with JIS B 0601, of 0.15 to 0.31 µm.